In the case where the cold working is performed as a cold-worked metal tube, i.e., a cold-finished steel tube, a proper surface treatment is performed to inner and outer surfaces of the steel tube to process the steel tube to have predetermined dimensions such that the rolling oil is applied during cold rolling or such that the steel tube is coated with a lubricant (metal soap) for cold drawing.
In the case where the cold-worked steel tube is subjected to the heat treatment, it is necessary that the rolling oil or the lubricant be washed (degreased) to remove the adhered substance to the inner and outer surfaces of the steel tube before the heat treatment. When the heat treatment is performed while the adhered substance remains on the surface of the steel tube, because sometimes the rolling oil or the lubricant contains chlorine and the like in addition to the hydrocarbon component, the rolling oil or lubricant is evaporated during the heat treatment to generate contaminant gases such as a chlorine gas, which sometimes results in the contamination on the inner surface of the steel tube where the contaminant gas remains particularly easily.
Even if the evaporated gas does not contain the contaminant gases such as the chlorine, sometimes carburizing occurs depending on temperature conditions because the inner and outer surfaces of the steel tube are exposed to a high-temperature gas containing a carbon source. In the steel tube in which the carburizing occurs on the surface thereof, SCC (stress corrosion cracking) is possibly generated from the carburized portion when the steel tube is used repeatedly in high temperature and high pressure. Therefore, in the case where the cold-worked steel tube is subjected to the heat treatment, it is necessary that the carburizing does not occur on the inner and outer surfaces of the steel tube.
When the adhered substance remaining in the inner and outer surfaces of the steel tube is to be removed before the heat treatment lest the contamination or carburization be generated on the surfaces of the steel tube during the heat treatment, a process for cleaning the inner surface such as sandblasting is additionally required because the adhered substance cannot be removed only by alkali degreasing and washing after the cold working. When acid pickling is applied, man-hours increase. In any case, costs for producing the steel tube increase due to the cold working.
Because it is effective to replace the gas inside the tube with an atmosphere-control gas to prevent the contamination or carburizing on the inner surface of the steel tube, conventionally various countermeasures are proposed.
In a tube-inside gas purging apparatus disclosed in Japanese Patent Application Publication No. 5-320745, a pair of opening/closing doors with elastic pads as opposed to each other is provided so as to independently be movable upward or downward, at the entrance of a purging chamber, incoming straight tubes are tentatively halted at the entrance and are pinched by actuating the upper and the lower doors to thereby increase a pressure of the atmosphere-control gas in the purging chamber to replace the gas inside the straight tubes with the atmosphere-control gas.
However, in the apparatus disclosed in Japanese Patent Application Publication No. 5-320745, a heat treatment efficiency is remarkably decreased due to the need to stop feeding of the straight tubes at the entrance of the purging chamber each time. At the same time, the elastic pad in the heated atmosphere is terribly deteriorated, which results in a problem in that required performance is not achieved or the elastic pad needs to be frequently exchanged.
In a heat treatment apparatus disclosed in Japanese Patent Application Publication No. 6-128645, a loading table for feeding the straight tube toward the entrance of the straight tube is provided at a side portion of a heat treatment furnace for performing the heat treatment for the straight tube in an atmosphere-control gas, and a negative-pressure applying means is provided in the loading table. The negative-pressure applying means causes a negative pressure onto rear ends of the straight tubes while front ends of the straight tubes enter into and reside inside the heat treatment furnace. Therefore, purging operation into the inside of the straight tube is extremely easily performed.
However, in the apparatus disclosed in Japanese Patent Application Publication No. 6-128645, the needs for the large-capacity negative-pressure applying means requires large-scale facility investment, which unfortunately increases the cost for producing the steel tube.
Japanese Patent Application Publication No. 2004-239505 discloses a continuous heat treatment furnace characterized in that a heat-resistant curtain is provided in a furnace entrance so as to cover the whole surface of the furnace entrance therewith and the steel tube is fed through the heat-resistant curtain. In the heat treatment furnace, because a decomposed gas (contaminant gas) generated from the adhered substance to the inner surface of the steel tube likely reside inside the steel tube, the atmosphere-control gas is caused to migrate in to the inside of the steel tube from the front end thereof to create a significant gas flow inside the steel tube.
Specifically, in charging the steel tube to be heat-treated into the furnace, when by heating up, a surface temperature of the front end side of the steel tube that enters earlier reaches 200 to 600° C., the residual adhered substance is decomposed to generate the hydrocarbon gas and the like. Compared with the outside of the continuous heat treatment furnace, the positive pressure is established in the atmosphere-control gas of the furnace by covering the furnace entrance to seal the furnace, desirably by covering opposite ends of the furnace, i.e., the furnace entrance and the furnace exit portion. Therefore, the gas flow can be created from the front end toward the rear end of the steel tube.
Accordingly, in charging the steel tube into the heat treatment furnace, the flow of the atmosphere-control gas is created from the front end toward the rear end of the steel tube while the adhered substance remaining the inner surface is decomposed and removed, so that the atmosphere-control gas can easily replace the gas inside the steel tube and the contamination or carburizing attributable to the decomposed gas of the adhered substance can be prevented without decreasing the heat treatment efficiency.
However, in the heat treatment furnace disclosed in Japanese Patent Application Publication No 2004-239505, when the rear end of the steel tube to be heat-treated enters the furnace (exactly, inside of the heat-resistant curtain), a pressure difference is diminished between the front end and rear end of the steel tube to stop the gas flow inside the steel tube, and the hydrocarbon gas or contaminant gas likely remains near the rear end. Therefore, it is necessary that the temperature be always controlled at the entrance of the furnace such that the entrance of the furnace becomes the temperature at which the adhered substance to the inner surface of the steel tube can be decomposed before the rear end of the steel tube enters the inside of the heat-resistant curtain. Therefore, there is the need of the heat treatment furnace in which the hydrocarbon gas or contaminant gas can surely be removed by a facile method.